A method of observing a specimen in a state where a space between the specimen and an immersion objective lens is filled with a liquid is known. FIG. 8 is a cross-sectional view of a protective member 21 as a lens cover for protecting an objective lens 20, which is disclosed in Patent Literature 1.
The protective member 21 includes a cap 22, a resin film 23, a fixing device 24 and an optical window 25. After the liquid is dropped into the protective member 21, the protective member 21 is attached to the objective lens 20, then, the specimen is observed through the optical window 25.